Active Phase Of Catalyst Research Articles

Performance and stability of Pd–Pt–Ni nanoalloy electrocatalysts in proton exchange membrane fuel cells

Pd–Pt–Ni nanoalloy catalysts have been synthesized by a polyol reduction method and characterized for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). The performance of the membrane-electrode assembly (MEA) fabricated with the Pd–Pt–Ni catalysts is found to increase continuously in the entire current density range with the operation time in the PEMFC until it becomes comparable to that of commercial Pt. The Pt-based mass activity of Pd–Pt–Ni exceeds that of commercial Pt by a factor of 2, and its long-term durability is comparable to that of commercial Pt within the 200h of operation. Compositional characterizations by energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) suggest a dealloyed active catalyst phase consisting of Pd-rich core and Pt-rich shell, formed by dissolution of Pd and Ni under the testing conditions. The surface catalytic activity of nanoparticles can be modified by the strain effect caused by lattice mismatch between the surface and core components. Transmission electron microscopy (TEM) observation of the MEA cross-section reveals that the Pd ions move into the Nafion membrane and even to the anode side and redeposit on reduction by hydrogen crossover. The deposition of Pd-rich PdPt particles mainly forms a band at the center of the membrane and along the cathode/membrane interface. On the other hand, the Ni ions ion-exchange with the protons in the Nafion membrane.

Effect of Alumina Support on the Formation of the Active Phase of Selective Hydrodesulfurization Catalysts Co-Mo/Al2O3

Two different commercial alumina supports were used for the preparation of selective hydrodesulfurization (HDS) catalysts Co-Mo/Al 2 O 3 . The effect of support properties on the HDS activity and selectivity of the corresponding Co-Mo catalysts were studied by X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, scanning transmission electron microscopy-energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. The HDS activity and selectivity were measured using a continuous flow fixed-bed microreactor. The Al 2 O 3 support with higher crystallinity and fewer hydroxyl groups had a moderate metal-support interaction, which resulted in an increase of the average slab length and stacking number of MoS 2 . This gave more formation of the active CoMoS phase and an increase in HDS activity and selectivity. ：以两种商用 Al 2 O 3 为载体, 制备了汽油选择性加氢脱硫催化剂 Co-Mo/Al 2 O 3 , 并采用红外光谱、X 射线衍射、N 2 吸附-脱附、透射电镜、扫描透射-能谱和 X 射线光电子能谱等手段系统研究了载体物化性质对催化剂活性相形成的影响. 结果表明, 表面羟基数量少和结晶程度高的载体与活性金属间相互作用减弱, 促进了 Mo 物种的硫化还原, 使 MoS 2 片晶的尺寸和层数增加, 且其硫化态催化剂上 CoMoS 活性位更多, CoMoS/MoS 2 比更大, 因而显著提高了相应 Co-Mo 催化剂加氢脱硫活性和选择性. An Al 2 O 3 support with higher crystallinity and fewer hydroxyl groups has a moderate metal-support interaction that increased the slab length and stacking number of MoS 2 , which gave more formation of the active CoMoS phase and an increase in Hydrodesulfurization activity and selectivity.

Investigation on the active phase of CoMo catalyst for selective HDS by low temperature in situ FT-IR

Abstract The CoMo/Al 2 O 3 catalysts with different metal loading were studied by low temperature in situ FT-IR using CO as probe molecule which appears to be a powerful method by giving rise to signals specific for unpromoted and promoted Mo sites. The result revealed that the increase of CoMoS phase on the catalyst surface improves the HDS activity and selectivity. The ratio of active site number of CoMoS and MoS 2 correlates linearly with HDS selectivity, which provides an effective tool for developing industrial selective HDS catalysts.

Potassium stabilization in β-K 2Fe 22O 34 by Cr and Ce doping studied by field reversal method

The stabilization of potassium in β-K 2Fe 22O 34, the active phase of styrene catalyst, by chromium and cerium doping (2 wt.%) is investigated by means of potassium ion thermal desorption. The kinetics for K + emission from undoped and Ce-, Cr-doped β-K 2Fe 22O 34 is studied with the field reversal technique in high vacuum conditions. Rate constants for K + desorption are determined at 600 °C (typical styrene production temperature) from the K + flux changes upon rapid electric modulation of the sample potential. The strong stabilization effect of both surface (Ce) and bulk (Cr) additives are observed by decrease in the rate constant of potassium ion desorption from 24.5 ± 3.6 s − 1 for undoped to 11.4 ± 2.1 s − 1 and 8.2 ± 2.8 s − 1 for Ce- and Cr-doped β-K 2Fe 22O 34, respectively. The stabilization effect is also reflected in the previously found increase in K + desorption energy from 2.33 ± 0.03 eV for β-K 2Fe 22O 34 to 2.86 ± 0.01 eV for Ce–β-K 2Fe 22O 34 and 3.47 ± 0.01 eV for Cr–β-K 2Fe 22O 34 . A model of the adsorbed sites for K atoms including the potential energy surfaces for their desorption account reasonably well for the observed changes in desorption rates with addition of Ce and Cr. The stabilization effect is discussed in terms of different distribution of Ce and Cr dopants in the material and blocking of different K desorption sites on the surface.

Formation of the Co/ZrO2 catalyst active phase in the Fischer—Tropsch synthesis

Thermoprogrammed reduction and X-ray powder diffraction were used to study the phase composition and specific features of the activation process of the Co/ZrO2 catalytic system. Results of the physico-chemical studies were compared with the data on activity and selectivity of catalysts in the Fischer—Tropsch synthesis. Reducing activation of the Co/ZrO2 catalyst with the syn-gas can be performed directly during the Fischer—Tropsch synthesis, avoiding the high-temperature step of reduction with hydrogen. The Co/ZrO2 catalytic system can be used for the development of catalysts for the Fischer—Tropsch synthesis, which do not require a separate step of the high-temperature reduction with hydrogen.

Characteristics of Au/HMS catalysts for selective oxidation of benzyl alcohol to benzaldehyde

Au/HMS catalysts are prepared by impregnation method and direct synthesized method and evaluated for the selective oxidation of benzyl alcohol. Among all the catalysts studied, Au/HMS catalyst prepared by impregnation method exhibits the best activity, the benzyl alcohol conversion of 42.9% and benzaldehyde selectivity of 95% can be achieved at 80 °C, 1 atm O 2 and existence of Na 2CO 3 aqueous solution. The Au/HMS catalysts are characterized by low-angle X-ray diffraction (XRD), wide-angle XRD, N 2 adsorption/desorption and transmission electron microscopy (TEM) techniques. The effects of prepared method, gold particle size and HMS structure are investigated in detail. The active phase of gold catalysts for selective oxidation of benzyl alcohol is explored based on the catalytic performance and characterization results. Moreover, the possible activation mechanism for selective oxidation of benzyl alcohol on Au/HMS catalysts is studied by means of CO and O 2 adsorption in situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS).

Effect of polyoxometalate amount deposited on Pt/C electrocatalysts for CO tolerant electrooxidation of H2 in polymer electrolyte fuel cells

Abstract Polyoxometalate-deposited Pt/C electrocatalysts are prepared by impregnation with various amounts of polyoxometalate (POM) anions (from 2 to 16.7 wt.% PMo12O403–) on the Pt/C catalyst. The prepared electrocatalysts show a high CO electrooxidation performance over a half-cell system for CO stripping voltammetry, and CO tolerant electrooxidation of H2 is further demonstrated over a proton exchange membrane fuel cell by using CO-containing H2 gas feeds (0, 10, 50, and 100 ppm CO in H2). In the CO stripping voltammograms, the onset and peak potentials for the CO oxidation appear to decrease as the POM deposition is increased, indicating that the electrooxidation of CO undergoes more efficiently on the catalyst surface with the deposited POMs on the Pt/C catalysts. In the single fuel cell tests with the CO-containing H2 gases, the higher current density is also generated with the larger amounts of deposited POMs on the Pt/C catalysts. Importantly, the charge transfer resistance Rp appears to decrease monotonically with the POM amounts, which was measured by electrochemical impedance spectroscopy. Physico-chemical characterizations with electrocatalytic analyses show that the deposited POMs hardly affect the active phase of Pt catalyst itself but can help the electrooxidation of H2 by efficiently oxidizing CO to prevent the Pt catalyst from poisoning. Consequently, this POM-deposited Pt/C catalyst can serve as a promising CO tolerant anode catalyst for the polymer electrolyte fuel cells that are operated with hydrocarbons-reformed H2 fuel gases.

Effect of surface properties of activated carbon on CO oxidation over supported Wacker-type catalysts

The effects of chemical pretreatments of the support on CO oxidation over the activated carbon-supported Wacker-type catalyst were studied. Dry oxidation by air at 400 °C and wet oxidation in H 2O 2 and H 3PO 4 aqueous solution were used to modify the physical and chemical properties of activated carbon. The textural and surface characteristics of activated carbons were characterized by nitrogen adsorption, temperature-programmed desorption (TPD) and Boehm titration. The dispersion of active phase and chemical environment of CO oxidation catalysts were investigated by X-ray Photoelectron Spectroscopy (XPS) and temperature-programmed reduction (TPR). CO oxidation results showed that the catalyst prepared using activated carbon with high mesopores and oxygen-functional groups as support performs higher CO oxidation activity at ambient temperature, especially in high relative humidity. XPS and TPR results show that the activated carbon support with enriched oxygen-functional groups would benefit the distribution of active components (Pd 2+, Cu 2+) on the catalyst surface and the formation of easily reducible Cu 2+ species.

In situ Mössbauer investigation of iron oxide catalyst in water gas shift reaction – Impact of oxyreduction potential and temperature

The aim of the study is to obtain the exact state of iron oxide catalyst active phase in reaction conditions, as well as the correlation between the active phase and catalytic properties of iron-containing catalysts. In situ Mössbauer spectroscopy is the major investigation technique. It is established that the change of reaction conditions (temperature and gas reaction mixture) lead to redistribution of the relative weight of spectra components and influence mainly tetrahedrally and octahedrally coordinated cations in Fe3O4 phase. It was concluded, that the active sites of the catalyst in studied reaction are probably pairs of Fe3++Fe2+-(Fe2.5+) ions, i.e. the mixed valance iron ions. The obtained catalytic activity can be explained with combination of the natural thermo-activated and catalytically induced electron exchange and better synchronizing of oxidation and reduction steps of the catalytic reaction.

Influence of additives in defining the active phase of the ethylene oxychlorination catalyst

The understanding, at the atomic level, of the role played by additives (dopants or promoters) in the chemistry of an industrial catalyst is a very complex and difficult task. We succeeded in this goal for the ethylene oxychlorination catalyst (CuCl(2)/gamma-Al(2)O(3)), used to produce dichloroethane, a key intermediate of the polyvinyl chloride chemistry (PVC). Among the most used additives for both fluid and fixed beds technologies (LiCl, KCl, CsCl, MgCl(2), LaCl(3), CeCl(4)) we have been able to highlight that KCl, and CsCl, forming in reaction conditions a mixed phase with CuCl(2), strongly modify the catalyst behaviour. In particular, these additives are able to displace the rate determining step from the CuCl oxidation (undoped catalyst) to the CuCl(2) reduction. This results from the decrease of the rate of the latter reaction, thus the overall activity of the system. For all remaining additives the rate determining step remains the CuCl oxidation, as for the undoped catalyst. These results have been obtained coupling the catalyst activity monitored with a pulse reactor working in both non-depletive and depletive modes with time resolved XANES spectroscopy performed under in operando conditions (i.e. coupled with mass spectrometry). Formation of CuK(x)Cl(2+x) and CuCs(x)Cl(2+x) mixed phases has been proved monitoring the Cu(II) d-d transitions with UV-Vis spectrometer and the CO stretching frequency of carbon monoxide adsorbed on reduced catalyst by in situ IR spectroscopy. Finally, of high relevance is the observation that the fully oxidized catalyst is inactive. This unexpected evidence highlight the role of coordinatively unsaturated Cu(I) species in adsorbing ethylene on the catalyst surface indicating that copper, in the working catalyst, exhibits a (I)/(II) mixed valence state.

K and Mn doped iron-based CO 2 hydrogenation catalysts: Detection of KAlH 4 as part of the catalyst's active phase

Hydrogenation of CO 2 to hydrocarbons is investigated over iron-based catalysts supported on γ-alumina to produce unsaturated hydrocarbons as feedstock chemicals for jet-fuel synthesis. Over 40% conversion levels can be achieved by doping this catalyst with Mn and K, along with an olefin/paraffin ratio of over 4. The doping levels played a crucial role in the product distribution as well as CO 2 conversion yields, with over doping leading to suppression of the desirable hydrocarbon products. The characterization of the catalyst showed the presence of KAlH 4 as part of the catalyst's active phase, acting as a reversible H 2 reservoir and as a center for H 2 activation. Characterization of the catalysts by XRD, XPS, and SEM sheds light on the role each dopant had on the overall catalyst's activity and product distribution.

Microreactor for Acetone Deep Oxidation over Platinum

AbstractA catalytic, silicon‐based, microstructured reactor was developed for the catalytic oxidation of volatile organic compounds, with the aim of small‐scale localized air treatment. The microreactor was based on the stacking of microstructured platelets, and the active catalyst phase, i.e., platinum, was deposited onto the walls of the microchannels by the thermal evaporation technique. The experimental setup allowed testing the efficiency of the catalytic microreactor. The deep oxidation of acetone (as a target compound) in air was carried out at increasing temperatures, resulting in ignition curves. Wide ranges of inlet acetone concentrations (500–8000 ppmv) and space velocities (18,700–201,000 h–1) were studied. Regarding acetone oxidation, the catalytic microreactor was found to exhibit high performances in terms of conversion, selectivity, and temperatures required to reach 50 and 95 % conversions.

A Rational Comparison of the Optimal Promoter Edge Decoration of HDT NiMoS vs CoMoS Catalysts

In order to gain a better understanding of the morphology and promoter edge content of the active phase of industrial HDT NiMoP catalysts in working conditions, a multi-technique study has been undertaken on a series of NiMoP catalysts with various Ni/Mo ratios. The combination of X-ray Photoelectron Spectroscopy (XPS), Transition Electron Microscopy (TEM), Density Functional Theory (DFT) modeling and catalytic testing (toluene hydrogenation) provided data to build a morphological model of NiMoS nanocrystallites. A parallel has been established with their CoMoS counterparts obtained in our previous work in order to emphasize differences arising from the promoter atom. This study confirms the importance of the presence of mixed Ni-Mo sites on the edges of the NiMoS nanocrystallites, and especially on the M-edge for reactions involving hydrogenation. These results provide new guidelines for future and ever more active catalysts.

Coordinatively Unsaturated Al 3+ Centers as Binding Sites for Active Catalyst Phases of Platinum on γ-Al 2 O 3

Bonding Oxides and Metals The binding of noble metals that can act as catalysts to metal oxides that are reducible is assumed to occur at the exposed cation of the oxide. For nonreducable oxides such as aluminum oxide, it is not so obvious how the metal can bind strongly. Kwak et al. (p. 1670 ) used a combination of high-resolution transmission electron microscopy and solid-state magic-angle spinning nuclear magnetic resonance to study the anchoring of platinum at high and low loadings on alumina. At the surface, the Al 3+ ions were penta-coordinated. Density functional calculations support a model in which the cation binds three oxygen atoms in the alumina and two from platinum oxide.

Studying PW-Amberlite catalyst deactivation in limonene epoxidation by hydrogen peroxide

The PW-Amberlite catalyst is active for limonene epoxidation in triphasic conditions; it becomes deactivated in reaction conditions. Catalyst stability during the reaction and recovery of catalyst activity when it was treated with several solvents were evaluated. It was found that the catalyst recovered 99% of its initial activity when it was washed with toluene and that the recovery was 95% and 97% when ethanol or acetone were used as washing solvents, respectively. Leaching tests showed that the reaction did not continue when the catalyst was removed from the reaction mixture, confirming the absence of leaching during the catalyst's active phase. FTIR analysis revealed that the characteristic species of the phosphotungstate complex did not vary with successive reusing of the catalyst.

Efficiency of catalytic processes for the reduction of CO and VOC emissions from wood combustion in domestic fireplaces

Pollutant characterization of domestic fireplaces, according to two paces of functioning (normal and low-charge phase) was performed. Two catalysts supported on cordierite or metal were placed in the exhaust of two domestic fireplaces (old and new generation) in order to reduce gaseous pollutants. Active phase of catalysts is composed of noble metals (Pd, Pt) and cerium. Methane was the dominant compound of the released Volatile Organic Compounds (VOC, 80% b. v.). Products resulting from incomplete combustion (CO and other VOC) did not represent more than 6% of the initial carbon content in wood. Lower concentration of CO in the exhaust was obtained with the new generation fireplace as compared to the older one with mean concentrations of CO normalized for 13% oxygen b.v. equal to 0.12% and 0.3%, respectively. Emission of VOC is also drastically reduced for new generation fireplace. The presence of a catalyst induced a decrease of the CO and VOC emission factors during ignition and low-charge phases by factors ranging from 65% to 70%. The abatement of VOC for the old generation fireplace was better in the presence of metal as compared to cordierite, with efficiency values of 65% and 30%, respectively. The new fireplace was the one on which the addition of the cleanup implements had most impact. Besides the introduction of a catalyst, a heating system of the fume was set up below the catalyst. This heating system allowed a faster activation of the catalyst, particularly during ignition and low-charge phases. Best abatements were obtained with the heated metallic support with values close to 80% and 94% for VOC and CO respectively.

Understanding Palladium Hydrogenation Catalysts: When the Nature of the Reactive Molecule Controls the Nature of the Catalyst Active Phase

Die prinzipiellen Unterschiede in der Pd-katalysierten Hydrierung von C-C-Doppel- und -Dreifachbindungen sind auf deutliche Unterschiede in der Zusammensetzung der Katalysatoroberfläche zurückzuführen. Die In-situ-Röntgenphotoelektronenspektroskopie des oberflächennahen Bereichs aktiver Pd-Katalysatoren enthüllt eine starke Korrelation zwischen der chemischen Natur (Alkin/Alken) der reagierenden Moleküle und dem Oberflächenzustand des Katalysators (I–IV).

Understanding Palladium Hydrogenation Catalysts: When the Nature of the Reactive Molecule Controls the Nature of the Catalyst Active Phase

Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany, Institute of Isotopes, Hungarian Academy of Sciences, Post Office Box 77, Budapest H-1525, Hungary WestCHEM, Department of Chemistry, University of Glasgow, Glasgow G128 QQ, Scotland, UK Universite de Lyon, Institut de Chimie, Laboratoire de Chimie, Ecole Normale Superieure de Lyon and CNRS 46 Allee d’Italie, 69364 Lyon, Cedex 07 (France)

Catalytic oxidation of cyanides in an aqueous phase over individual and manganese-modified cobalt oxide systems

The possibility for purification of wastewaters containing free cyanides by applying of a new method based on cyanides catalytic oxidation with air to CO 2 and N 2 at low temperature and atmospheric pressure was investigated. On this purpose, individual and modified with manganese Co-oxide systems as active phase of environmental catalysts were synthesized. The applied method of synthesis favours the preparation of oxide catalytic systems with high active oxygen content (total-O* and surface-O* s) possessing high mobility, and the metal ions being in a high oxidation state and in an octahedral coordination-factors determining high activity in reactions of complete oxidation. The catalysts employed were characterized by powder X-ray diffraction, Infrared spectroscopy, and chemical analysis. The effect of pH of the medium and catalyst loading on the effectiveness of the cyanide oxidation process, expressed by the degree of conversion ( α, %), by the rate constant ( k, min −1), and COD was studied. The results obtained reveal that using catalysts investigated a high cyanide removal efficiency could be achieved even in strong alkaline medium. The higher activity of the manganese promoted catalytic sample could be explained on the basis of higher total active oxygen content and its higher mobility both depending on the conditions, under which the synthesis of catalyst is being carried out.

The Nanoscience Revolution: Merging of Colloid Science, Catalysis and Nanoelectronics

The incorporation of nanosciences into catalysis studies has become the most powerful approach to understanding reaction mechanisms of industrial catalysts and designing new-generation catalysts with high selectivity. Nanoparticle catalysts were synthesized via controlled colloid chemistry routes. Nanostructured catalysts such as nanodots and nanowires were fabricated with nanolithography techniques. Catalytic selectivity is dominated by several complex factors including the interface between active catalyst phase and oxide support, particle size and surface structure, and selective blocking of surface sites, etc. The advantage of incorporating nanosciences into the studies of catalytic selectivity is the capability of separating these complex factors and studying them one by one in different catalyst systems. The role of oxide–metal interfaces in catalytic reactions was investigated by detection of continuous hot electron flow in catalytic nanodiodes fabricated with shadow mask deposition technique. We found that the generation mechanism of hot electrons detected in Pt/TiO2 nanodiode is closely correlated with the turnover rate under CO oxidation. The correlation suggests the possibility of promoting catalytic selectivity by precisely controlling hot electron flow at the oxide–metal interface. Catalytic activity of 1.7–7.2 nm monodispersed Pt nanoparticles exhibits particle size dependence, demonstrating the enhancement of catalytic selectivity via controlling the size of catalyst. Pt–Au alloys with different Au coverage grown on Pt(111) single crystal surface have different catalytic selectivity for four conversion channels of n-hexane, showing that selective blocking of catalytic sites is an approach to tuning catalytic selectivity. In addition, presence and absence of excess hydrogen lead to different catalytic selectivity for isomerization and dehydrocyclization of n-hexane on Pt(111) single crystal surface, suggesting that modification of reactive intermediates by the presence of coadsorbed hydrogen is one approach to shaping catalytic selectivity. Several challenges such as imaging the mobility of adsorbed molecules during catalytic reactions by high pressure STM and removing polymeric capping agents from metal nanoparticles remain.